Method of forming turbine blade root

ABSTRACT

The present invention relates to a process for forming the root of a turbine blade using the EDM method. EDM machining of the root is done in a manner that any re-cast layer left behind is less than 1 micron. After EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks. A liquid tracer coating is then applied to the root, which allows the ability of the next process to be verified as complete. The root is then subjected to glass beading to remove any re-cast and insure the surface finish prior to shot peening is consistent and contains no scratch marks or machine marks. The root is then subjected to shot peening to reduce residual tensile stresses by imparting to the surface small indentations or dimples and produce a compressed surface which resists further surface cracks.

FIELD OF THE INVENTION

[0001] The present invention relates to a process for making turbine blades for use in power generation, aerospace, the pumping industries and the like. In particular it relates to a process for forming the root of the turbine blade by a combination of wire electrical discharge machining (EDM) and subsequent glass beading and shot peening of the machined portion.

BACKGROUND OF THE INVENTION

[0002] The turbine blade, for example, in the jet engine, has a very simple purpose, to re-direct and compress air. The turbine blade is attached to a ring, along with a varying number of identical turbine blades. When the turbine blades are all installed to the ring, they form a perfect circle. Several stages of rings are mounted, one behind the other, each with a smaller diameter as you get to the hotter section of the turbine. As the rings are rotated, each blade moves air using the principal of an airfoil and/or bucket. As the air is passed on to the next stage, the area, or volume decreases, thereby compressing the air into a smaller and smaller area. Once the air finally reaches the required volume, it is mixed with fuel and ignited causing, in the case of a gas turbine, a massive controlled explosion, which is then directed in some cases into another ring of turbine blades.

[0003] The turbine blade has a blade or foil, a root and a shroud or tenon. The root allows the blade to be affixed to the rotating disk or ring. The root is a critical feature, as the root must be a near perfect fit in order to prevent separation from the disk. During the operation of a turbine, the roots are subjected to high residual tensile stresses. Therefore the blade root must be cut with precision in order to achieve a near perfect fit, at the same time maintaining durability.

[0004] The current methods of manufacturing a root are generally broken down into three categories: grinding, broaching or conventional machining using profile milling and/or a form tool. The turbine blade and root are usually made from stainless steel ranging in many different grades. Blades, while less common, can also be made from super alloys and ceramics with high nickel content. It is for this reason that machining or removing material to form the root is extremely difficult. The cost of these conventional machining methods are high.

[0005] When forming a root by grinding, a blank is mounted into a CNC (computer numerical controlled) grinder. The root shape is ground in with a grinding wheel having the opposing shape of the root to be formed.

[0006] Broaching is a process of cutting with a tool that consists of a bar having a single edge or a series of cutting edges (i.e. teeth). A blank is mounted into a broaching machine. The broach cuts in a straight line or axial direction relative to the motion of the workpiece. The entire cut is made in a single or multiple passes over the workpiece to shape the required surface contour. Some of the disadvantages of broaching are:

[0007] High tooling cost

[0008] Grinding costs are high

[0009] Machine cost is extremely high

[0010] Foot print of the machine is large

[0011] Tremendous cutting pressure (tears rather than cuts)

[0012] Setup time for each operation is extensive

[0013] Tool lead time is great

[0014] Machining with a form tool is one of the most common practices. A form tool is a cutting tool, which which produces its inverse or reverse form counterpart upon a workpiece (i.e. the form tool has the desired shape of the root). The form tool is used in various styles of machining centers, and is spun at the required RPM (revolutions per minute) and moved perpendicular into the blank/workpiece to machine in the root. Some of the disadvantages of the form tool are: high tooling costs and tool lead time is great.

[0015] Manufacturing the root with a broach or a form tool may raises problems. In both instances, if the broach or the form tool breaks or malfunctions in the middle of a production run, it is nearly impossible to reproduce or repair the broaching tool or form tool to the identical shape. This will cause mis-match in a production run. Further, if a form tool or a broach breaks, and deliveries are critical, in almost every circumstance unless a spare tool is on premises, a new tool can take a long period of time to acquire. Also 99% of the time the part cannot be repaired.

[0016] When supplying blades to the service industry, many times with only a used broken part as the pattern to manufacture new pieces, reverse engineering a component is many times the only practical method. The root will be mounted into a ring that has lost a varying amount of material, and the blade has to fit onto that ring. The practice is common to supply the service industry with a range of sample parts to fit onto the ring, in order to achieve the best fit. If one were to produce these samples with a broaching tool or a form tool, the cost to supply a great number of samples would be tremendous. This is due in part from the fact that a set of form tools and or broaching tools would require to be ground for each sample.

[0017] The methods of machining a root with broaching, grinding or use of a form tool, all are susceptible to wear. These three processes do not have any means of compensating for tool wear. Subsequently, the manufacturer must rely on frequently changing the tool or redressing the grinding wheel in order to keep the process stable.

SUMMARY OF THE INVENTION

[0018] It is an object of the present invention to provide a process for forming the root of a turbine blade that overcomes the problems associated with tool malfunctions in the middle of a production run.

[0019] It is a further object of the present invention to provide a process for forming the root of a turbine blade that can provide a range of sample parts to fit onto the ring quickly and cost effectively.

[0020] It is a further object of the present invention to provide a process for forming the root of a turbine blade that eliminates problems associated with tool wear.

[0021] It is a further object of the present invention to provide a process for forming the root of a turbine blade that is associated with exotic/advanced materials.

[0022] Accordingly the present invention provides a process for forming the root of a turbine blade using electrical discharge machining (EDM) comprising the following steps:

[0023] a. EDM machining of the root into the desired shape in a manner that any re-cast layer left behind is less than 4 micron.

[0024] b. After EDM machining the turbine blade can optionally be subjected to magnetic particle inspection (MPI) to check for depth or presence of surface cracks.

[0025] c. A liquid tracer coating is preferably then applied to the root, which allows the ability of the next step to be verified as complete.

[0026] d. The root is then subjected to glass beading to remove any re-cast and insure the surface finish prior to the next step is consistent and contains no scratch marks or machine marks.

[0027] e. The root is then subjected to shot peening to reduce residual tensile stresses by imparting to the surface small indentation or dimples and produce a compressed surface which resists further surface cracks. The peening is preferably done using 110 steel shots at an intensity of 0.005A2-0.007A2.

[0028] Further features of the invention will be described or will become apparent in the course of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order that the invention may be more clearly understood, the preferred embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:

[0030]FIG. 1 shows microstructural analysis of an EDM section for a 400 series stainless steel piece under 500× magnification with a recast layer <4 μm using a rough pass.

[0031]FIG. 2 shows microstructural analysis of an EDM section for a 400 series stainless steel piece under 1000× magnification with a recast layer <1.5 μm using a trim pass.

[0032]FIG. 3 is a cross section of a turbine root using a rough pass that has been subjected to glass beading.

[0033]FIG. 4 shows metallographs taken from the locations marked 1 (left), 2 (middle) and 3 (right) in FIG. 3.

[0034]FIG. 5 is an enlarged illustration of shot peening a surface.

[0035]FIG. 6 shows compressed grains trying to restore the original shape of the surface producing a hemisphere of cold-worked metal highly stressed in compression.

[0036]FIG. 7 shows the effect of shot peening on a crack in a surface.

[0037]FIG. 8 shows the SN-curve for samples tested under different processes: Conventional Machining (CM), Electrical Discharge Machining (EDM), and EDM followed by Shot Peening (EDM+SP).

[0038]FIG. 9 shows how the incremental angle for a radial root is calculated.

[0039]FIG. 10 shows a turbine blade with a male root.

[0040]FIG. 11 shows a turbine blade with a female root.

[0041]FIG. 12 shows how radial drops are calculated.

[0042]FIG. 12A shows an enlarged analysis of the area circled on FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The electrical discharge machining (EDM) method, according to the present invention, of forming the turbine blade root is very much different from conventional methods. All other methods of removing the material effectively rip, tear, or score the material. EDM is a thermal erosion process in which metal is removed by rapid spark discharge between an electrode and a conductive workpiece. In the case of a wire EDM, a series of discharges occur between the wire electrode traveling longitudinally through the workpiece. This discharge occurs in a voltage gap between the electrode and the workpiece. The relative moving passage between the wire electrode and workpiece is controlled by a CNC system according to a programmed CNC program to cut the workpiece into desired shapes. Heat from the discharge melts or vaporizes minute particles from the workpiece. However the EDM method does leave behind a thin “recast” layer where EDM has altered the metallurgical structure of the workpiece. While manufacture of the blade using EDM is uncommon, processes to EDM the root have been proposed but have never been commercially acceptable. This is in part due to residual tensile stresses developed on the surface of the machined area of the metal. In high stress turbine applications these can lead to crack propagation, which could result in failure of a blade root.

[0044] The type of surface finish and residual recast layer thickness depends on many parameters such as, number and/or type of passes (rough pass vs. trim pass), cutting speed, cutting current, time delay, etc. The process of the present invention eliminates near all recast form the EDM process prior to MPI and shot peening. In the present invention, EDM machining of the root from a 400 series stainless steel work piece is done in a manner that any re-cast layer left behind is negligible, generally less than 4 μm (<0.00016″) using a rough pass and preferably <1 μm (0.00004″) can be achieved using trim passes, an erosion or cutting speed of approximately 2 mm/minute, an erosion or cutting current of 18@22 pulses per second, and a time delay of 55 units. FIGS. 1 & 2 show microstructural analysis of an EDM section for a 400 series stainless steel piece under high magnification using a rough pass vs. a trim pass. After EDM machining, the root of this turbine blade is subjected to MPI to check for depth or presence of surface cracks. A liquid tracer coating is then applied to the root, which allows the ability of the next process to be verified as complete. The root is then subjected to glass beading to remove any re-cast. By glass beading the surface prior to shot peening, the surface finish prior to shot peening is consistent and contains no scratch marks or machine marks, which are always evident after conventional machining. FIG. 3 shows a cross section cut from a turbine root (rough pass) that has been subjected to glass beading. The cut piece was mounted and polished. The EDM machined surface layer (at the locations framed and marked 1,2,3) of the part was analyzed using metallographic method, and the metallographs taken from the three locations are shown on FIG. 4. FIG. 4 shows that after glass beading the surface of the EDM recast later has been reduced to approximately 2 μm (<0.00008″). The root is then subjected to shot peening. The effect of this step ultimately produces a compressed surface which resists further surface cracks.

[0045] In known processes of making turbine blade roots after machining, the root is subjected to: MPI (magnetic particle inspection) and shot peening. The turbine blade is subjected to MPI to check for depth or presence of surface cracks. Shot peening is a cold working process where the surface of a part is bombarded with small spherical balls called shots (FIG. 5). When a small metal is struck by shots small indentations occur on the surface of the metal compressing the grains below the surface. The compressed grains try to restore their original shape of the surface producing a hemisphere of cold-worked metal highly stressed in compression (FIG. 6). Cracks will not initiate nor propagate in a compressively stressed zone and therefore shoot peening increases the fatigue life of the part and reduces failures due to crack propagation (FIG. 7). Shot peening of the material can be achieved using 110 steel shots at an intensity of 0.005A2-0.007A2, a standard peening intensity.

[0046] The EDM process produces many surface flaws/imperfections such as discontinuities, sharp edges etc. these surface imperfections are stress risers and produce tensile residual stresses, which may cause failure under certain loading conditions. Surface enhancements such as, polishing, glass beading, shot peening, etc. are required after EDM to reduce the tensile stresses and increase the fatigue life. FIG. 8 shows the SN-curve for samples tested under different processes; Conventional Machining (CM), Electrical Discharge Machining (EDM), and EDM followed by Shot Peening (EDM+SP). Shot peening dramatically increased the fatigue life of the samples.

[0047] The EDM process, of the present invention, to form a root of a turbine blade accomplishes the following objectives:

[0048] The root of all the turbine blades manufactured fit uniformly within the mating ring. As noted previously, manufacturing the root with a broach or a form tool raises problems, if the broach or the form tool breaks in the middle of a production run. Using these methods it is nearly impossible to reproduce or repair the broaching tool or form tool to the identical shape. This will cause mis-match in a production run. Further, if a form tool or a broach breaks, and deliveries are critical, in almost every circumstance unless a spare tool is on premises, a new tool can take a long period of time to acquire. By using the EDM process of the present invention, tool breaks do not create a problem. Replacement electrodes will produce identical pieces and are readily available.

[0049] Blades supplied to the service (repair) industry are not generally made to a single specification. Also, when supplying blades to the service industry, many times only a used broken part is provided and reversing engineering a component is the only practical method of manufacture. The root will be mounted into a ring with has lost a varying amount of material, and the new blade has to fit onto that ring. The practice is to supply the service industry with a range of sample parts to fit onto a ring, in order to achieve the best fit. If one were to produce these samples with a broaching tool or a form tool, the cost would be higher to supply a large number of samples. This is due in part from the fact that a set of form tools and or broaching tools would require to be ground. The EDM process of the present invention eliminates the need for a large number of form tools or broaching tools. The same EDM machine can be programmed to produce different roots to accommodate ring wear.

[0050] When machining a root with broaching, grinding or use of a form tool, the tools all are susceptible to wear. These three processes do not have any means of compensating for tool wear. Subsequently, the manufacturer must rely on frequently changing the tool or redressing the grinding wheel in order to keep the process stable. The EDM process of the present invention to form the root of the turbine blade is not susceptible to tool wear. The wire being used in the process is continuously wound through and disposed.

[0051] In known processes of making turbine blade roots after machining, the root is subjected to: MPI (magnetic particle inspection) and shot peening. The turbine blade is subjected to MPI to check for depth or presence of surface cracks. By shot peening, the material is effectively bombarded with small media called shot. Each piece of shot striking the material acts as a tiny ball peen hammer, imparting to the surface small indentation or dimples. The effect of this process ultimately produces a compressed surface which resists further surface cracks.

[0052] The process of the present invention eliminates any re-cast from the EDM process prior to MPI and shot peening. In the present invention, EDM machining of the root from a stainless steel workpiece is done in a manner that any re-cast layer left behind is less than 4 micron. After EDM machining, the turbine blade is subjected to MPI to check for depth or presence of surface cracks. A liquid tracer coating is then applied to the root, which allows the ability of the next process to be verified as complete. The root is then subjected to glass beading to remove any re-cast. By glass beading the surface prior to shot peening, the surface finish prior to shot peening is consistent and contains no scratch marks or machine marks, which are always evident after conventional machining. The root is the subjected to shot peening to impart to the surface small indentation or dimples. The effect of this step ultimately produces a compressed surface which resists further surface cracks.

[0053] The process of the present invention is applicable to forming a turbine blade root in a completely vertical cut (straight cut) or in a similar fashion using multiple cuts at varying angles in order to form radial surfaces.

[0054] A circle and/or radius can be created by simply drawing tangent lines perpindicular to the radius (FIG. 9).

[0055] The accuracy of the circle/radius is dependent on the incremental angle θ where the tangents are drawn. For the purpose of the present invention, the distance between the tangents intersection point “F” and the surface of the circle is called the cordial deviation “ψ”. The cordial deviation is a constant and is directly proportional to the customer tolerance (root clearance). As a standard we will use ψ=• customer clearance tolerance. (0.0003 will be used as a constant).

[0056] In the blade industry, generally there are two types of blade roots, male roots (FIG. 10) and female roots (FIG. 11). The EDM machine can be programmed to cut the radial roots in two different methods depending on the type of root. For a male root ψ₊=Superior cordial deviation (deviation above the surface) is used. For a female root ψ⁻=Inferior cordial deviation (deviation below the surface) is used.

[0057] The incremental angle used for programming is calculated as follows: $\frac{\Delta \quad {EDF}}{\angle \quad {EDF}} = {90{^\circ}}$

[0058] R=Radius from centre of rotor to radial loading surface ${{{Cos}(\varphi)} = \frac{R}{L}},{L = \left( {R + \psi_{+}} \right)}$ $\varphi = {{Cos}^{- 1}\left( \frac{R}{L} \right)}$

[0059] Superior Deviation (Male root): Draw tangent ⊥ to R at ∠2φ increments starting at apex.

[0060] Inferior Deviation (Female root): Draw tangent ⊥ to H at ∠φ increments from apex, H=(R−ψ⁻).

[0061] The calculated incremental angles are then inputted into the EDM machine to machine the radial loading faces of the root. Since most blade roots have more than one loading face, an incremental angle for each loading face need be calculated.

[0062] A second “Radial Drop” calculation is required for final inspection. This is to ensure that the calculated incremental angle & program are correct and that the final results is within customer specification.

[0063] Radial Drops are calculated as follows (FIG. 12):

[0064] β=Packing angle

[0065] c=distance from staking axis to packing face

[0066] D=distance from centre of rotor to c

[0067] T=Radial Drop

c=R Sin(β)

R ² =D ² +c ²

D={square root}{square root over (R²−c²)}

T=R−D

T=R(1−{square root}{square root over (1−(sin β)²)})

[0068] Although various preferred embodiments of the present invention have been described herein in detail, it will be appreciated by those skilled in the art, that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A process for forming the root of a turbine blade comprising the following steps: (a) EDM machining of the root into the desired shape in a manner that any re-cast layer left behind is less than 4 micron or greater if acceptable; (b) After EDM machining, the root is then subjected to glass beading to remove any re-cast and insure the surface finish is consistent and contains no scratch marks or machine marks; and (c) Then the root is subjected to shot peening to reduce residual tensile stresses by impart to the surface small indentations or dimples and produce a compressed surface which resists further surface cracks.
 2. A process according to claim 1 wherein the process forms the straight root of a turbine blade.
 3. A process according to claim 1 wherein the process forms the straight root of a turbine blade.
 4. A process according to claim 2 wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.
 5. A process according to claim 3 wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.
 6. A process of forming the radial root of a turbine blade comprising of the following steps: a. Calculation of the incremental angle at which tangents are drawn to produce a radial root. b. Inputting data corresponding to the radial portion of the root blade into an EDM apparatus. c. EDM machining of the root into the desired shape in a manner that any re-cast layer left behind is less than 4 micron or greater if acceptable; d. Calculation of the radial drop in order to inspect the radial drop on the root. e. Glass beading of the root to remove any re-cast and insure the surface finish is consistent and contains no scratch marks or machine marks; and f. Shot peening of the machined portion of the root to reduce residual tensile stresses.
 7. A process of forming the radial root of a turbine blade according to claim 6 wherein the calculation in step (a) is for a female root
 8. A process of forming the radial root of a turbine blade according to claim 6 wherein the calculation in step (a) is for a male root.
 9. A process according to claim 6 wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.
 10. A process according to claim 7 wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.
 11. A process according to claim 8 wherein additional steps after EDM machining and before glass beading are provided, namely: after EDM machining the turbine blade is subjected to MPI to check for depth or presence of surface cracks; and then a liquid tracer coating is applied to the root.
 12. A process according to claim 1 wherein the re-cast layer is less than 1 micron.
 13. A process according to claim 6 wherein the re-cast layer is less than 1 micron.
 14. A turbine blade having a root formed by the process of any of claims 1 to
 13. 